Molding method and cooling apparatus for pressure resisting bottles of synthetic resin

ABSTRACT

In order to improve the productivity of pressure-resisting bottles by reducing the blow time from 4-5 seconds in a conventional bottle molding method to not more than 3 seconds, and mold pressure resisting bottles free from the occurrence of crazing and bottom-breakage, a biaxially stretched blow molded bottle (1) of a synthetic resin having a big foot type petaloid bottom is produced in a blow time of not more than 3 seconds, and after the product has been released from the mold, the bottom portion (3) has become not higher than 70° C., whereby the shortage of the capability of the metal mold of cooling the bottom portion (3) is supplemented. This allows both the prevention of the irregular deformation of the central part (6) of the bottom portion, and the molding of the bottom portion (3) of high pressure resisting strength and mechanical strength with a sufficient height H of the central part (6) of the bottom portion ensured.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to molding methods and a cooling apparatusfor pressure resistant bottles of synthetic resin, especiallypolyethylene terephthalate resin (hereinafter called PET).

2. Background of the Invention

Recently, biaxially stretched big PET bottles, such as 1.5 literbottles, are widely used. These bottles are big foot type petaloidbottles which are the only bottles that have self-standing bottlebodies. Prior bottles are composed of a bottle body having a roundbottom portion and a base cup attached to the round bottom portion. Thebottom portion of the big foot type petaloid bottle has a complexstructure to enable itself to be self-standing and to resist an innerpressure. The big foot type petaloid bottle does not require a base cup,so that, compared to the prior bottle, it has higher productivity andless scrap problem after use.

The distribution of wall thickness of a 1.5 liter pressure resistingbottle is shown in FIG. 4. The central part 6 (a range of 40 φ of theundersurface of the bottom portion 3) is thicker than the thickness ofthe leg portion 3 and the valley portion 5 as shown by line a. Thethickness of the central part 6 is more than 2 mm.

The central part 6 of the bottom 3 is required to be thick. When thecentral part 6 is thin, it lacks mechanical strength and induces crazingand bottom-breakage. This is caused by the high temperature and pressuregenerated after a substance is filled in the bottle.

Therefore, the bottom part 6 of the bottle should be thicker compared toother parts of the bottle. However, since the bottom part 6 is thicker,it takes longer for the bottom part 6 to cool to a certain temperature(normally, to the temperature of glass transition point of the syntheticresin). When the blowing time is short and the center part 6 is notcooled enough, the center part 6 of the bottle projects outwardly, asshown in FIG. 8, after being released from the mold.

In the past, to prevent deformation of the center part 6, blowing timeis set to more than 4 seconds to cool the center part 6 by a blow mold.The temperature characteristic curve b in FIG. 5 shows a relationshipbetween blow time and temperature of the center part 6, which ismeasured 7 seconds after a bottle is released from a blow mold. It isclear from this temperature characteristic curve b that to preventdeformation of the center part 6 and to cool it below the glasstransition point, blow time of longer than 4 seconds is required.

FIG. 6 shows a variation characteristic of blow time which variesdepending on the height H (see FIG. 8) between the bottom edged of theleg portion 4 and the center undersurface of the center part 6. In FIG.6, the characteristic curve C1 indicates that 1 second of blow time isapplied, c2 indicates 2 seconds of blow time, c3 indicates 3 seconds ofblow time, c4 indicates 4 seconds of blow time, c5 indicates 5 secondsof blow time and c6 indicates 7 seconds of blow time. From experience,crazing and breakage are prevented for a height H of 4.0 mm or more.According to FIG. 6, more than 4 seconds of blow time are required.However, if more than 4 seconds of blow time are applied, productivityfails to improve.

To increase productivity, a number of ideas is discussed, such as toimprove the cooling capacity of the blow mold or to make the wall of ablow mold thinner, thereby affecting the cooling agent to the mold faceof the blow mold. However, such ideas do not efficiently cool the centerpart of a bottle while raising the cost for equipment.

Therefore, the object of the invention is to shorten the blow time andimprove productivity. Further, the mechanical strength of the bottomportion of the bottle is improved.

SUMMARY OF THE INVENTION

The invention provides a molding method for producing pressure resistantbottles of synthetic resin by blowing the bottle in a blow mold forabout 2 seconds. Within 4 seconds after the bottle is released from theblow mold, the center part of the bottom portion of the bottle iscompulsively cooled below about 70° C. within a time of 5.5 seconds to7.0 seconds.

Another molding method for producing the pressure resistant bottles ofsynthetic resin comprising a big foot type petaloid bottom is asfollows: 1) Blow mold the bottle with a blowing time of about 1.5seconds to 3.0 seconds. 2) Within 15 seconds after the bottle isreleased from the blow mold, the projecting deformation of the centerpart of the bottom portion formed after being released from the blowmold is forced onto a reform jig and is pushed back to the maximumoriginal position. Concurrently, the inside of the bottle is pressurizedand the center part is cooled below about 70° C.

Another molding method for producing the pressure resistant bottles ofsynthetic resin comprising a big foot type petaloid bottom is to blowmold the bottle with a blowing time of about 1.5 seconds to 3.0 seconds.Within 15 seconds after the bottle is released from the blow mold, thecenter part of the bottom portion is forced onto a double-form jig toraise the height of the center portion above the original height fromthe blow mold while the inside of the bottle is pressurized and thecenter part is cooled below about 70° C.

The invention provides a cooling apparatus for pressure resistantbottles of synthetic resin having a big foot type petaloid bottom forcooling the bottom immediately after the bottle is released from a blowmold. The cooling apparatus includes a reform jig comprising a moldface. A projecting height of a center part of the mold face is similarto the height of the center part of a bottom mold of a molding device.Other portions of the reform jig are also similar to the molding device.A jet mouth for cooling air is formed at the center part of the moldface. A press jig, capable of holding the bottle and cooperating withthe reform jig, presses the bottom portion onto a mold surface of thereform jig and a cooling nozzle, capable of entering inside the bottle,blows air onto the upper surface of the bottom portion and pressurizethe inside of the bottle.

The invention provides another cooling apparatus for producing thepressure resistant bottles of synthetic resin having a big foot typepetaloid bottom for cooling the bottom immediately after the bottle isreleased from a blow mold. The cooling apparatus includes a double-formjig comprising a mold face. A projecting height of the center part ofthe mold face is higher than the height of the center part of a bottommold of a molding device. Other portions of the double-form jig aresimilar to the molding device. A jet mouth for cooling air is formed atthe center part of the mold face. A press jig, capable of holding thebottle cooperating with the double-form jig, presses the bottom portionto a mold surface of the double-form jig and a cooling nozzle capable ofentering inside the bottle blowing air onto the upper surface of thebottom portion and pressurizing the inside of the bottle.

The function of the present invention will be described below.

As shown in FIG. 6, even if only 2 seconds of blow time is applied,within 4 seconds after released from the blow mold, the height H of thebottle remains more than 4 mm. Therefore, by cooling a bottle with 2seconds of blow time, and by compulsively and rapidly cooling the bottomthereof within 4 seconds after being released from the blow mold, theheight H of the center part of the bottom portion of the bottle ishigher than 4 mm.

By arranging the compulsive rapid cooling of the bottom portion of thebottle so that the temperature of the center part of the bottom portiondecreases below 70° C. within 5.5 to 7.0 seconds, deformation of thecenter part caused by high heat (about 80° C. when measured) isprevented. The heat comes out of the surface after 25 to 30 secondsafter being released from the mold. In addition, cooling and hardeningof the bottom portion can be completed.

When the blow time is 1.5 to 3.0 seconds, the height becomes less than 4mm within 15 seconds after being released from the blow mold. Even 15seconds after being released from the blow mold, the height of thebottom portion continues to decrease. This indicates that within 15seconds after a bottle is released from the blow mold, the center partof the bottom portion can be deformed by an outer force.

According to various experimental tests, it is determined that thecenter part of the bottom portion of a bottle can be deformed to adesired shape by an outer force only when the deformation is appliedwithin 15 seconds after the bottle is released from the mold because ofa cooling effect in the atmosphere.

By forcing the center part of the bottom portion of the bottle, whichprojects outwardly, to a reform jig within 15 seconds after release, theprojected center part returns to the blow molded position, the maximumreturning position, by repulsive force of the reform jig and then cooledand hardened at that position.

By applying pressure inside the bottle while the bottom portion thereofis forced onto the reform jig, the rest of the bottom portion of thebottle, such as leg and valley portions, do not deform because of thepressure inside the bottle. The compulsive cooling of the bottom portioncools the center part below 70° C. within the shortest period of time,however, the cooling time is not limited to any range.

By increasing the height of the bottom portion, the resistant strengthof the bottom portion of the bottle can be improved. However, when theheight is set higher by arranging a blow mold, an extension ratio of theleg portion increases so that the bottle cannot be as easily formed andvoids are produced.

To avoid the above effects, the height of the bottom portion is blowmolded a little lower than the final product. After releasing the bottlefrom the mold, the center part is deformed by a double-form jig insteadof the reform jig until the center part comes up to the final productposition and is cooled and hardened.

The above process provides a compulsive shape of the bottom portion thatis smoothly blow molded. The height can be maintained high enough, sothat the bottle is resistant against high pressure and the bottom doesnot induce breakage.

A cooling process using the cooling apparatus attaches a bottle releasedfrom a blow mold to a reform jig. The bottom portion of the bottle isformed by the form surface of the jig and the mouth portion of thebottle is pushed downward by a press jig. The center part of the bottomportion of the bottle is reformed to the original shape and height, themaximum height, formed in the mold.

While pressing the mouth portion of the bottle by the press jig, coolingair is provided through the cooling nozzle. The upper surface of thebottom portion of the bottle is cooled and the inside of bottle ispressurized. At the same time, the cooling air is flowed from the flowmouth of the reform jig and cools the undersurface of the bottom portionand hardens the center part thereof. The center part of the bottomportion returns to the original shape formed in the mold.

When a double-form jig is applied instead of a reform jig, a similarprocess as described above is used, and the height of the bottom portionis pushed upward by the jig to be higher than the height previously setby the blow mold.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail with reference to the followingdrawings, wherein:

FIG. 1 shows a partial elevation view of a preferred embodiment;

FIG. 2 shows a sectional view of a double-form jig;

FIG. 3 shows a sectional view of a bottom portion of a bottle reformedby a double-form jig shown in FIG. 2;

FIG. 4 shows a graph of a distribution of a wall thickness of a bottomportion;

FIG. 5 shows a graph of temperature characteristic of a center part ofthe bottom portion against blow time of a bottle;

FIG. 6 shows a graph of various characteristics of the height accordingto various blow time after release from the mold;

FIG. 7 shows a graph of a breakage ratio of the bottom portion accordingto the reform ratio; and

FIG. 8 shows a sectional view of a deformation of the bottom portion ofa bottle because of the lack of cooling after being released from themold.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments according to the present invention will bedescribed below referring to the drawings.

FIG. 1 shows a partial elevation view of a cooling apparatus accordingto the present invention, wherein a shaft 15 is secured to a base block14 in a standing position. A holder 23 which holds a mouth portion 7 ofa bottle 1 and a handler 24 which holds a body 2 is fixed to the shaft15. A cooling block 18, positioned below the handler 24, is secured tothe base block 14.

On the cooling block 18, there is a reform jig 9 comprising a moldsurface 10 having a projecting central portion. The projecting centralportion is arranged similar to the form surface of a bottom mold of ablow molding device. The other portions of the mold surface 10 is alsoarranged similar to the blow molding device. The reform jig 9 is cooledby the cooling water W that flows through a cooling passage 22 providedin the cooling block 18. A passage block 19 forms an air cooling passage21 of the cooling block 18 and runs in the center of the reform jig 9 inthe downward direction. The upper surface comprising recess 20 for thecooling air A to escape is positioned at the center of the mold surface10 of the reform jig 9. The front edge of the air cooling passage 21 ispositioned as a jet mouth of the cooling air A.

A cooling nozzle 17 is fixed to an upper portion of the shaft 15 and iscapable of moving in upward and downward directions. A press jig 16 isfixed to the cooling nozzle 17. The press jig 16 comprises passagesthrough which the air in the bottle escapes when the jig 16 is airsealed by the mouth portion 7 of the bottle.

The cooling nozzle 17 moves in the downward direction by a cylinder (notshown) and enter in the bottle 1 through the mouth portion 7. The pressjig 16 is arranged such that when the bottom edge of the cooling nozzle17 comes close to the center part 6 of the bottom portion 3, it pushesthe upper surface of the mouth portion 7.

The cooling block 18 is capable of moving downwardly and upwardly for adistance greater than the height of the mold surface 10 of the reformjig 9. The holder 23 and the handler 24 lightly hold the bottle 1 toprevent the bottle 1 from moving laterally and enable the bottle 1 to bemoved in the downward and upward directions.

FIG. 1 shows the bottle 1 released from a mold 7 seconds prior to beingattached. The bottle 1 is pressed by the press jig 16 with 25 kg/f, andcooled by the cooling air A of 6.5 kg/cm² supplied through the coolingnozzle 17 and the cooling passage 21. By keeping the inner pressure ofthe bottle at 6 kg/cm², the center part 6 of the bottom portion 3 iscompulsively cooled for 5 seconds, while the cooling air A is releasedthrough two passages of 1.5 mm φ of the press jig 16.

Tests are applied to bottles which have received 1.5 seconds, 2.0seconds, 2.5 seconds and 3.0 seconds of air blow. The bottles which arecompulsively cooled according to the present invention, the heights H ofthe bottom portions of the bottles 1 are all 4.2 mm. On the other hand,bottles that have not been compulsively cooled, the heights H are 2.0 mmwith 1.5 seconds blow, 2.3 mm within 2.0 seconds blow, 2.7 mm with 2.5seconds blow and 3.2 mm with 3.0 seconds blow. All of them are under 4.0mm.

Tests for determining breakage of the bottom portion of the bottle isalso done using bottles blown for 2.5 seconds of blow time. For 20bottles that received the compulsive cooling according to the presentinvention, no bottle breakage occurred. In contrast, breakages occurredin 18 out of 20 bottles that have not been compulsively cooled.

FIG. 2 shows a center face 13 of a mold surface 12 of a double-form jig.The mold surface 12 is below the center face 13 as shown by the dottedlines. Thus, the center face 13 projects higher than the mold surface 12of the bottom mold of the blow molding device. Therefore, the bottomportion 3 of the bottle 1 completed by the double-form jig 11, as shownin FIG. 3, has the center part 6 recessed inwardly when compared to thedotted line. The curvature of the leg portion 4 towards the insidebecomes smaller and the curvature of the valley portion 5 becomeslarger.

The reformation of the center part 6 has limitations. The bottle 1 isblow molded for 2 seconds of blow time in a blow mold having the bottommold height of 3.9 mm. Five seconds after being released from the mold,the bottle 1 is reformed and cooled in the cooling apparatus 8 havingthe double-form jig 11, as shown in FIG. 1. When the double-form jig 11has a height of 5 mm, the reform quantity is 1.1 mm, the height thereofis 4.43 mm and the mold ratio is 114%. When the double-form jig 11 has aheight of 6 mm, the reform quantity is 2.1 mm, the height thereof is5.49 mm and the mold ratio is 141%.

Further, when the double-form jig 11 with the height of 7 mm is applied,the reform quantity is 3.1 mm, the height is 5.86 mm and the reformratio is 150%. The reform ratio is calculated by the formula: (theheight of the reform bottom portion 3/the height of the bottom mold of ablow mold device)×100.

Breakage tests are performed using the reformed bottles mentioned above.When a double-form jig 11 having a height of 5 mm is used, breakagesoccur in one out of 15 bottles. For a double-form jig 11 having a heightof 6 mm, breakages occur in 9 out of 15 bottles. For a double-form jig11 having a height of 7 mm, breakages occur in 12 out of 15 bottles.

As shown in FIG. 7, the allowable breakage ratio of the prior art is30%. For 30% breakage ratio, the mold ratio of the present invention isnearly 130%. As the forming capability of the present biaxial blowmolding method and device are advanced, the mold ratio of the presentinvention may be limited to below 120%.

The Effects of the Invention

The present invention provides the following features:

The bottom portion of the bottle being biaxially stretch molded iscooled immediately after it is released from the mold, so that theblowing time of the blow mold can be shortened and the productivityimproved.

The bottom portion of the bottle after blow molding is compulsivelycooled after the bottle is released from the mold. Thus, a new blow moldis not required and bottles are produced with lower cost equipment.

With a reform jig 9 or a double-form jig 11, the heights of the bottomportion of the bottle is maintained sufficiently high so that bottleswith higher resistance against pressure and outer force can be produced.

The cooling apparatus comprises a reform jig 9, or double-form jig 11, apress jig 16 and a cooling nozzle 17. Thus, the cooling apparatus issimple and it handles and operates easily.

We claim:
 1. A cooling apparatus for cooling pressure resisting bottlesmade of synthetic resin, each of the bottles having a big foot typepetaloid bottom, said apparatus being provided for cooling the bottom ofeach of said bottles after said bottles are released from a moldingdevice, said apparatus comprising:a reform jig having a mold face, themold face having a center part with a projecting height that issubstantially equal to a height of a center part of a bottom mold of themolding device, said reform jig further comprising a jet mouth forproviding cooling air to the center part of said mold face; a press jigcapable of holding said bottles, said press jig being cooperable withsaid reform jig to press said bottom of each of said bottles to the moldface of said reform jig; and a cooling nozzle that enters inside each ofsaid bottles, blows air toward an upper surface of said bottom of eachof said bottles and provides pressure inside each of said bottles.
 2. Acooling apparatus for cooling at least one pressure resisting bottle ofsynthetic resin, each of said at least one bottle having a big foot typepetaloid bottom, said apparatus being provided for cooling the bottom ofeach of said at least one bottle after said at least one bottle isreleased from a molding device, said apparatus comprising:a double-formjig comprising a mold face, the mold face having a center part with aprojecting height that is higher than a height of a center part of abottom mold of the molding device, said double-form jig furtherincluding a jet mouth for providing cooling air to the center part ofsaid mold face; a press jig capable of holding each of said at least onebottle, said press jig being cooperable with said double-form jig topress said bottom of each of said at least one bottle to the mold faceof said double-form jig; and a cooling nozzle that enters inside each ofsaid at least one bottle, blows air to an upper surface of the bottom ofeach of said at least one bottle and provides pressure inside each ofthe at least one bottle.